The outlined proposal is directed to a better understanding of the processes of determination, differentiation and pattern formation. These processes are common to all developing systems and therefore most likely have similar underlying mechanisms controlling them. We plan to study these problems during embryogenesis and metamorphosis in Drosophila, crucial stages during development. By altering normal interactions between egg regions during early development by ligation we will ascertain whether these interactions play a similar role in the development of the nervous system as they do in the development of the larval and adult epidermis. By halting cell division in embryos, the requirement of cell division for the differentiation of the nervous system can be determined. By analyzing the position and fate of dividing cells in regenerating fragments of imaginal discs, we will learn more about the mechanisms by which cells acquire, maintain, and alter commitments during development. Studies of spatial and temporal heterogeneity of the developing leg disc will help determine the role that these factors play in normal development. By studying the patterns of replacement of antenna structure by leg structure in the homoeotic mutant Antennapedia and by observing the regulative behavior of fragments of wild type and leg and antenna discs, we can determine to what extent pattern formation and pattern regulation are based on similar mechanisms.